In certain transducers of the prior art, one side of a supporting annular ring is clamped in cantilever fashion between two opposed stators. A pair of flexures extend inwardly from the opposite side of the ring to support a disklike proof mass. The proof mass includes a torque coil mounted on each face, which upon displacement of the proof mass, operates to restore the mass to a centered position relative to the stators. Surrounding the coil is a plated pick-off capacitance area. Electrical paths on the flexures connect the torque coil and pick-off capacitance area to leads on the support. A representative example of such a transducer is described in greater detail in U.S. Pat. No. 4,250,757, assigned to the same assignee as the present invention.
A problem related to such transducers arises when a force is applied to the supporting ring in a direction perpendicular to the plane of the ring, causing the ring to deflect. The linear and angular deflection of the supporting ring is translated through the flexures to the proof mass, causing a centroid of capacitance, i.e. the effective center of the pick-off capacitance area for small displacements, to be displaced from its normal position wherein it is approximately centered between the top and bottom stators. The torque coil reacts to the displacement of the centroid of capacitance by restoring the proof mass to its prior position. However, because there has been a repositioning of the proof mass resulting in bending of the flexures, a continuous restoring torque is required to balance the moment applied by the flexures. Consequently, the output signal from the transducer includes a bias shift component.
External imbalanced forces applied to the proof mass supporting structure can result from a variety of causes. For example: (a) the gold fly wires that connect to the support may exert a residual force which relaxes over time due to the creep characteristics of gold; (b) an elastic damping material applied to the support may produce an imbalanced force on the structure, due to thermal variations in the environment; (c) static charge buildup can produce either an attractive or repulsive force between the support and an adjacent surface; and, (d) preload variations and thermally variable distortion may result should the cantilevered portion of the support contact an adjacent part of the stator through a contaminating particle or due to assembly error.
A dynamic source of force imbalance applied to the proof mass support may result from loading the support with a "g" force (force of acceleration). In this instance, the bias shift is a linear function of the acceleration, and thus appears as a shift in the transducer scale factor. Such an apparent shift in the scale factor occurring over time can create a significant problem when the transducer is exposed to vibration at a frequency near the resonant frequency of the support. The overall effect of such a dynamically induced loading on the support manifests itself as a vibration rectification error at certain frequency ranges.
Whether resulting from static or dynamically induced imbalanced loading, deflection of the supporting element can cause an undesirable bias shift or error signal in the output of transducers of the prior art type described above. The present invention seeks to compensate for deflection of the support due to such force, whatever its cause, and thereby to minimize bias shift and dynamic signal error in the transducer output that might otherwise result.